254 BUTTER-MAKING. 



out the coarse organic and inorganic matter held in suspension, 

 without removing the bacteria. If some bacteria are removed 

 with the matter held in suspension, others are carried along 

 from the filter-bed. Owing to this, a new filter-bed must be 

 kept in operation a few days before the filtered water can be 

 considered pure and ready for use. The following table illus- 

 trates how the germ-content of water is decreased as the process 

 of filtration is carried on during the first few days : 



It will be seen from the table that during the first three 

 days the filter-bed was in use the filtered water contained more 

 germs than the unfiltered. Good results were not obtained 

 until the seventh day. In order to be on the safe side it is 

 best to expose the filter-bed to continuous filtration for about 

 nine days before the water is used. 



The slimy coat referred to above is formed by certain germs. 

 These germs then constitute the real agent of filtration. In 

 order for these micro-organisms to do efficient work oxygen 

 is essential. Well-water of average purity contains enough 

 oxygen in solution without employing an intermittent process of 

 filtration, and consequently for creamery purposes the con- 

 tinuous method of filtration is to be recommended. 



Intermittent. The intermittent process of filtration is used 

 where comparatively impure water is being purified, such as 

 in purifying \vater for large cities. If the continuous process 

 of filtration were employed in such instances, the filtered 

 water would not be free from germs, due to the fact that impure 

 river-water does not carry enough oxygen in solution to supply 

 the germs which form the real filtering agency. 



